2.2 Quartiles from a Simple Frequency Table

If your data set is very small, it’s easy enough to line them up in order and count out the positions of the quartiles. But what happens if there are 50 or 100 numbers and not just a dozen or so?

Example 2.2a
The table below shows how many pairs of shoes of different sizes a shop has.
Find the median, quartiles and semi-interquartile range.